It depends on how large each site is. Generally, I would allow each C5 install to have at least 512 MB of storage (concrete itself takes up around 30MB), so in your case you could comfortably host maybe 100 C5 installs. Again, it depends on how much space you think you'll need for each install.

Personally, I would allow 1 GB per install, just for sake of flexibility.

50 GB SSD Diskspace as PineCreativeLabs said you *might* be able to get 100 pretty much bare bones installs on there if none of them were ever serving up any sites. Many sites we host are around the 1 GB mark by the time you get all content and images in there, especially if you have novice users uploading their raw quality images to their website. However, 1 CPU is most likely going to be your point of contention long before anything else. You'll certainly run into slow performance if you get any kind of traffic against the server. I'd honestly be surprised if that VPS was meant to be sold for anything more than serving up a single site (or maybe a few test websites).